Field of the Invention
The present invention relates to aerobic wastewater treatment systems and, more particularly, to an aerobic treatment/clarifier tank for use in such system.
Description of Prior Art
Aerobic wastewater treatment systems (AWTS) are generally comprised of a pre-treatment tank wherein most of the solids settle out, an aerobic treatment/clarifier tank (AC tank) wherein wastewater from the pre-treatment tank is aerobically treated and clarified and a pump or holding tank into which clarified, aerobically treated wastewater flows from the AC tank for further disposal. There are numerous AWTS systems available, the details of operation and construction of which are well understood by those skilled in the art.
A typical AC tank comprises an aerobic chamber and a clarifying chamber, both of which are formed inside a housing, vessel or the like. For example, in one common form of AC tank, there is a generally cylindrical vessel or housing in which is disposed an inverted, frustroconical partition, the partition dividing the vessel into an aerobic chamber and a clarifying chamber, the clarifying chamber being within the partition. In AC tanks of this design, i.e., wherein there is a sloped wall defining at least a portion of the clarifying chamber, the slope angle of the sloped wall surface is important from an operational point of view. For example, if the slope angle is too shallow, undigested solids in the clarifying chamber will settle on the sloped wall surface, rather than falling under the force of gravity back to the aeration chamber where they can be further aerobically digested. Accordingly, regulatory agencies, as for example, state health departments, together with other parameters of the AC tank, specify a range of the slope angle of the sloped wall so as to minimize adhesion of the undigested solids on the sloped wall.
In theory, the steeper the slope angle of the sloped wall the less the problems encountered with settling of undigested solids on the sloped wall surface. However, slope angles which are too steep also present problems. Because the capacity requirements of the clarifying chamber and the aerobic treatment chamber are also relevant considerations, the slope angle of the sloped wall has to be maintained in a range, generally of from 50° to 70°, preferably from about 55° to about 65°, more preferably about 60°, lest the capacity balance between the clarifying chamber and the aerobic treatment chamber be outside acceptable parameters, thus interfering with the performance of the AC tank. Furthermore, a clarifying chamber having a steep slope angle requires an AC tank which has greater height than an AC tank having a sloped wall of a shallower angle, given that in either case, the AC tank meets the capacity parameter balance for the clarifying chamber and the aerobic treatment chamber.
As is well known to those in the industry, AWTS systems are typically buried in the ground such that the upper surface of the AWTS is substantially flush with grade, with the exception that access hatches into the system extend above grade for maintenance, inspection, etc. This means that if the AC tank has a sloped wall defining at least a portion of the clarifying chamber which is steep, to keep within an acceptable capacity ratio of the clarifying chamber and the aerobic treatment chamber, a deeper hole must be dug to accommodate the AWTS. This presents installation problems.
There are certain areas where the AWTS is to be installed, wherein rock; e.g., granite, is reached at a relatively shallow depth below the overburden. In these areas, an AWTS with an AC tank having a sloped wall, which has a steeper slope angle (making the tank taller), may necessitate jack hammering the bottom of the hole or excavation into which the AWTS is to be placed to remove the rock, e.g., granite, so that the AWTS can be installed with the top surface substantially at grade. Obviously, in these circumstances, an AWTS having an AC tank which is shorter and accordingly has a shallower slope angle is desirable since the total height of the AWTS may be such that it can be buried in the overburden above the rock subsurface without having to jack hammer or otherwise remove the rock. Yet in still other areas, a deeper hole can pose other problems. For example, on land which is adjacent water; e.g., islands, and wherein the elevation of the land relative to the adjoining body of water is relatively small; e.g., 3 ft. to 5 ft., the high level of the water table can exert hydrostatic force against the AC tank, which can cause the AWTS, in certain instances, to float. Furthermore, in many of these areas, where the soil is sandy, the sides of the hole or excavation into which the AWTS is to be placed will continue to collapse, meaning that an excavation of a much larger cross-sectional area must be made in order to obtain a substantially level surface upon which the AWTS can be set. Furthermore, in general, it is well known in the industry that installers do not like to install AWTS's requiring deep holes or excavations as opposed to AWTS's requiring shallow holes or excavations.